In a conventional priority control using a single output queue, because its output order is fixed, if a convergence detecting signal is input into a head packet, such a phenomenon that a subsequent packet cannot be output although it can be output because it does not receive a convergence detecting signal often occurs.
Because of this phenomenon, a sufficient capacity for storing successively input packets is necessary.
According to conventional technologies disclosed in for example, Japanese Patent Application Laid-Open No. 7-135512, Japanese Patent Application Laid-Open No. 10-65733, Japanese Patent Application Laid-open No. 10-243016 and the like, an increase of the capacity is suppressed by discarding an input packet early (Early Packet Discard method) or discarding a packet in a single input queue at random (Random Early Discard method).
Further, although a solution method in which plural queues are possessed for each output path has been proposed, separate controls for each path are necessary, so that a complicated control is required.